This invention relates to printing using a thermal print head in which elements of the head are selectively heated by electric current to cause transfer of ink from a thermal ink transfer ribbon to a medium on which printing is to be effected.
Thermal printers are known which comprise a row of print elements consisting of thin or thick film resistors deposited on a substrate. Usually the resistors are formed as a single resistive strip and the individual resistive printing elements are defined by electrical connections to the strip at spaced points along the length thereof, the portion of strip between two adjacent connection points forming an element. A thermal ink transfer ribbon consisting of a film carrying a layer of ink is positioned between the print elements and a medium on which printing is to be effected, the ink layer being adjacent the medium. A pressure roller urges the medium into contact with the ink layer and the ribbon into contact with the print elements so that when any print element is heated by electric current passed therethrough, by way of the electrical connections defining that element, the ink layer in the region of that heated element is softened and caused to adhere to the surface of the medium. The medium and the ribbon are fed past the row of print elements and during this movement the print elements are selectively and repeatedly heated such as to cause a desired pattern to be printed row by row. The quality of the printing obtained is dependent to some extent upon the properties of the surface of the medium and if the medium has a rough surface the transfer of ink from the ribbon to the medium may be non-uniform and as a result the quality of the printing may be poor. In many situations where thermal print heads are utilised in a thermal transfer printing process it is possible to specify the physical properties required for the surface of the medium and thereby ensure attainment of the desired quality of printing. However in machines for printing postal franking on mail items it is not possible to ensure that the surface of all mail items will have the required properties. Generally envelopes have a satisfactory surface but users of franking machines may decide to use envelopes having a surface on which it is difficult to attain a desired high quality of printing. It is a requirement that postal franking machines should be capable of printing satisfactorily on all types of envelopes available to the user of the machine.